first commit of the new libm!

[musl] / src / math / ilogb.c

diff --git a/src/math/ilogb.c b/src/math/ilogb.c
new file mode 100644 (file)
index 0000000..c5915a0
--- /dev/null
+++ b/src/math/ilogb.c
@@ -0,0 +1,21 @@
+#include <limits.h>
+#include "libm.h"
+
+int ilogb(double x)
+{
+       union dshape u = {x};
+       int e = u.bits>>52 & 0x7ff;
+
+       if (!e) {
+               u.bits <<= 12;
+               if (u.bits == 0)
+                       return FP_ILOGB0;
+               /* subnormal x */
+               // FIXME: scale up subnormals with a *0x1p53 or find top set bit with a better method
+               for (e = -0x3ff; u.bits < (uint64_t)1<<63; e--, u.bits<<=1);
+               return e;
+       }
+       if (e == 0x7ff)
+               return u.bits<<12 ? FP_ILOGBNAN : INT_MAX;
+       return e - 0x3ff;
+}